The present invention relates to treating fuel for use in aircraft and other vehicles, processor and more particularly to an apparatus and process for removing ice from refrigerated jet fuel.
Conventional fuel tanks for aircraft have ullages which contain explosive mixtures of atmospheric air and fuel vapors during various stages of operation of the aircraft. Conventional jet fuels include Jet- A, Jet A-1, Jet-B, JP-4, JP-5, JP-7, JP-8 and JP-8-100, as well as others, are loaded into aircraft fuel tanks. The fuel tanks of aircraft are vented to atmosphere to relieve ascent and descent pressure changes, allowing the atmospheric air to enter into the ullages and mix with the fuel vapors above the liquid levels of the volatile fuels contained within the fuel tanks. These explosive mixtures are highly flammable and are easily ignited by a spark or other ignition source, which can result in massive explosions and loss of life to those onboard the aircraft.
Previous attempts have been made to reduce the risks of explosion within the non-vented ullages of aircraft fuel tanks. One such attempt fills the ullages with an inert gas, such as nitrogen (N2), to blanket the liquid fuel and prevent air from entering the ullages so that the fuel vapors in the ullages will not have a readily available oxygen source. The inert gases were provided by on-board tanks or generating systems which proved inefficient and ultimately impractical due to the size of large, heavy tanks used for on-board storage or generating of the inerting gases, and the heavy logistics support required for such systems. This type of system also required alteration of existing aircraft to accommodate such equipment.
Another problem which exists is that the range of aircraft, or the distance which the aircraft can fly, is limited due to the limited volumetric storage capacity of onboard fuel tanks. While in-flight refueling is possible today with some types of aircraft, most refueling is performed on the ground. Such refueling stops prolong travel time and ruin the economics of operating an aircraft. It is therefore desirable to increase the amount of fuel that can be stored on the aircraft, preferably with no or only minimal alteration to the aircraft structure. On such method is that set forth in PCT International Patent Application Ser. No. PCT/US97/04091, filed on Mar. 17, 1997, entitled xe2x80x9cREFRIGERATED FUEL FOR ENGINES,xe2x80x9d invented by Terence Lee Koethe, published on Oct. 9, 1997 and claiming a priority date of Mar. 18, 1996, as set forth above in the section entitled xe2x80x9cCross Reference To Related Applications,xe2x80x9d and to which the present application claims priority as a Continuation-in-Part.
Reduced temperature fuels will also result in vapor pressures of the fuel vapors within the ullages of fuel tanks which are much lower than the vapor pressures provided by fuels of higher temperatures. Reducing the vapor pressures of fuel vapors within the ullages results in significantly reducing the explosive nature of the vapors in the ullages. The cooling of fuels to temperatures below ambient temperatures, and in particular, below the freezing point of water, thus functions as a vapor phase inhibitor in such ullages. Aircraft may be fueled with fuels which are specifically processed to such lowered temperatures, which will enhance both the safety and the performance of such aircraft.
Water has been a troublesome contaminant of airport jet fuel supplies. Water is found in fuel either in the form of dissolved water or free water. Dissolved water is provided by water molecules that are in solution with the fuel. Free water in fuels is typically in the form of either bulk quantities, such as a water slug, or as entrained water. Entrained water is typically in the form of very small droplets of water suspended in the fuel. Free water which is carried into the aircraft fuel system can cause operational difficulties. Small amounts of entrained water can be tolerated in turbine aircraft engines, typically amounts of less than 30 ppm. However, there is little margin for error and as a result flight performance is often limited in terms of both altitude and minimum allowed fuel temperature. Also, when low temperatures are encountered, such as the temperatures found at high altitudes, the free water in fuels may freeze, forming ice. Most aircraft are equipped with fuel warming devices which counteract the formation of ice to prevent possible flow constrictions caused by such ice in fuel.
Since water is heavier than fuel, the water settles out over time and water slugs are usually found at the bottom of large storage tanks. Floating siphons in storage tanks remove fuel from the upper regions of the storage tanks to assure that the water is not transferred to the aircraft as a water slug. However, some entrained water will still be removed from storage tanks by the floating siphons and pass into fuel flow lines. Entrained water may often be removed from the fuel flow lines by coalescing filters. Dissolved water molecules cannot be removed from fuels by filtration. However, with a reduction in the temperature of the fuel, dissolved water molecules are removed from solution within the fuel and become free water. Generally water comes out of solution at a rate of 1 ppm/xe2x88x92xc2x0 F. Additionally, as the temperature of the fuel is lowered below that of the freezing point of water, entrained water in the fuel will change phase from a liquid to a solid. At higher altitudes where lower temperatures are encountered, dissolved water may leave from solution with the fuel and become free water that will freeze and form ice particles within the fuel, which may impede fuel flow. At sub-freezing temperatures, entrained water molecules aboard aircraft in both free and dissolved form cause greater fuel viscosity, therefore, it is desirous to create jet fuel which is substantially free of dissolved water and entrained water.
Jet fuel is typically delivered to aircraft at temperatures above the freezing point of water, even during cold winter weather operations. The fuel is passed through conventional filtration equipment, which typically includes three stages of filtration that remove particulate matter and free water from the fuel. Particulate contamination is filtered out and liquid water is coalesced in a first stage of the conventional filtration equipment. In the second stage, free water is entrapped. The third stage typically includes monitoring devices to assure the fuel is of a desired quality. The monitors in the third stage will shut down the entire fuel loading process should the fuel quality not meet the desired quality standards. If fuel is loaded onboard aircraft at temperatures below the freezing point of water, ice will be formed by the free water and some of the dissolved water, which leaves solution with the fuel after cooling and becomes free water. This ice may be removed by the standard filtration equipment. Low temperature jet fuel operations, such as for extending the range of aircraft and for reducing the explosive nature of the ullages of aircraft, may cause excessive ice production from the free water and the water initially dissolved in aircraft fuels, such that the capacity of conventional filtration equipment will be exceeded and automatic failsafe system shutdowns will occur.
In one aspect of the present invention, a method and apparatus are provided for removing ice from low temperature aviation fuels. The temperature of the fuel is lowered beneath the freezing point of water, such that significant amounts of dissolved water leaves solution and becomes free water, and then the free water freezes and becomes filterable from the fuel. The fuel is then passed through a cyclonic separator, which spins a fuel in an intense cyclonic spiral, and centrifugal force separates the ice from the fuel. Heating elements are provided within the cyclonic separator to prevent blockage of ice and water discharge ports. A mixture of fuel, ice and water is passed from the cyclonic separator and into a reclamation unit, in which gravity separates a reclaimed portion of the fuel from the ice and water. The fuel is then passed through conventional aircraft fuel filtration equipment.
In another aspect of the present invention, a method and apparatus are disclosed for providing inert loading jet fuel for use in cold fuel, hot fuel or conventional temperature fuel applications for aircraft. Inert loading jet fuel is preferably provided by direct contact injection, in which an inerting material, such as, for example, nitrogen (xe2x80x9cN2), is directly injected into the jet fuel as the fuel is being loaded aboard the aircraft. The inerting material is preferably injected directly into the jet fuel by injection nozzles which are in direct contact with a flow of the jet fuel. The inert material is metered to accommodate an entrained level of the inerting material which will outgas as an inert gas into a ullage of an onboard, vented fuel tank of the aircraft, rendering the mixture of gases in the ullage nonexplosive for a period of time. The inerting material can be injected into fuels which are used to provide cold temperature fuels, which are cooled to temperatures that are less than ambient temperatures so that the unit volume of the fuel per pound of fuel is reduced. The fuel is preferably cooled externally to the aircraft at a ground location, and then stored in onboard fuel tanks at the reduced temperatures, allowing more fuel to be held in the storage tanks and increasing the energy value of the fuel per unit volume over fuel at ambient temperatures. A heat transfer surface may also cooled below ambient temperature using the cold fuel or the inerting material, and then ambient air is passed over the heat transfer surface and cooled for passing into the passenger compartment of the aircraft.